A fuel cell is a device capable of providing electrical energy from an electrochemical reaction, typically between two or more reactants. Generally, a fuel cell includes two electrodes, called an anode and a cathode, and a solid electrolyte disposed between the electrodes. The anode contains an anode catalyst, and the cathode contains a cathode catalyst. The electrolyte, such as a membrane electrolyte, is typically ionically conducting but electronically non-conducting. The electrodes and solid electrolyte can be disposed between two gas diffusion layers (GDLs).
During operation of the fuel cell, the reactants are introduced to the appropriate electrodes. At the anode, the reactant(s) (the anode reactant(s)) interacts with the anode catalyst and forms reaction intermediates, such as ions and electrons. The ionic reaction intermediates can flow from the anode, through the electrolyte, and to the cathode. The electrons, however, flow from the anode to the cathode through an external load electrically connecting the anode and the cathode. As electrons flow through the external load, electrical energy is provided. At the cathode, the cathode catalyst interacts with the other reactant(s) (the cathode reactant(s)), the intermediates formed at the anode, and the electrons to complete the fuel cell reaction.
For example, in one type of fuel cell, sometimes called a direct methanol fuel cell (DMFC), the anode reactants include methanol and water, and the cathode reactant includes oxygen (e.g., from air). At the anode, methanol is oxidized; and at the cathode, oxygen is reduced:CH3OH+H2O→CO2+6H++6e−  (1)3/2O2+6H++6e−→3H2O  (2)CH3OH+3/2O2→CO2+2H2O  (3)As shown in Equation (1), oxidation of methanol produces carbon dioxide, protons, and electrons. The protons flow from the anode, through the electrolyte, and to the cathode. The electrons flow from the anode to the cathode through an external load, thereby providing electrical energy. At the cathode, the protons and the electrons react with oxygen to form water (Equation 2). Equation 3 shows the overall fuel cell reaction.